In recent years, an individual identification technique using wireless communication with an electromagnetic field, an electric wave, or the like has attracted attention. In particular, an individual identification technique which employs an RFID (radio frequency identification) tag as a semiconductor device that wirelessly communicates data has attracted attention. The RFID tag is also referred to as an IC tag, an IC chip, an RF tag, a wireless tag, or an electronic tag.
RFID tags can be classified into two types: an active RFID tag that includes a power supply necessary for the circuit operation accompanying transmission and reception of data to/from an external wireless communication device (hereinafter referred to as a reader/writer), and a passive RFID tag that is driven by generating electric power inside the RFID tag by using electronic power of electromagnetic waves from a reader/writer. It is well known that a large number of passive RFID tags typified by Suica, PASMO, and ICOCA (which are registered trademarks) come on the market.
FIG. 16 illustrates an example of a wireless communication system using a passive RFID tag. The wireless communication system is constituted by a reader/writer 1601, a control terminal 1603, and a passive RFID tag 1605. The control terminal 1603 controls the reader/writer 1601. Data is wirelessly transmitted and received between an antenna 1602 connected to the reader/writer 1601 and an antenna 1606 of the passive RFID tag 1605.
Wireless data transmission and reception are performed as follows. A wireless signal output from the antenna 1602 connected to the reader/writer 1601 is received by the antenna 1606 in the passive RFID tag 1605. The wireless signal is an electromagnetic wave which is modulated in accordance with the data to be transmitted. An electromagnetic wave for transmitting data is referred to as a carrier wave. A wireless signal is also referred to as a carrier wave which is modulated in accordance with data. A wireless signal (hereinafter simply referred to as a carrier wave) 1604 is received by the antenna 1606 and input to a signal processing circuit 1609 to be processed through a power supply circuit 1607 and a demodulation circuit 1608 in the passive RFID tag 1605. In such a manner, the passive RFID tag 1605 obtains data included in the carrier wave 1604. Then, a signal containing response data is output from the signal processing circuit 1609. The passive RFID tag 1605 transmits the carrier wave 1604 corresponding to the signal to the antenna 1602 connected to the reader/writer 1601, through a modulation circuit 1610 and the antenna 1606 in the passive RFID tag 1605. The carrier wave 1604 corresponding to the signal is received by the antenna 1602, and the reader/writer 1601 obtains the response data and the response data is stored in the control terminal 1603.
The power supply circuit 1607 in the passive RFID tag 1605 generates a direct-current power supply voltage for driving the signal processing circuit 1609, based on the carrier wave received by the antenna 1606. Examples of a specific configuration of the power supply circuit 1607 are a half-wave rectifier circuit, a half-wave voltage doubler rectifier circuit, and a full-wave rectifier circuit.
Moreover, the demodulation circuit 1608 in the passive RFID tag 1605 demodulates components of a modulation signal included in the carrier wave received by the antenna 1606 and generates a demodulation signal for driving the signal processing circuit 1609. For such a passive RFID tag, improvement in capability of reading a wireless signal from a reader/writer has been actively researched (e.g., see Patent Document 1).